Himalayan Knotweed

Height: Grows up to 60 – 200 cm tall from creeping rhizomes.   

Leaves: Mature leaves are alternate, lanceolate (long and pointed) and 9 – 22 cm long, with bases that are slightly heart-shaped to tapered. They have stiff hairs on the leaf edges and are tapered at the end.  

Stems: Numerous, densely hairy, ribbed, red-brown, erect and branching.  Stems are hollow and have jointed nodes surrounded by a papery stipule. Stems can be up to 3 cm in diameter.  

Flowers: Are pinkish white and perfect flowers (both male and female organs). Seeds are smooth, shiny and black, 3 mm long and encased in a papery fringed capsule.  Seed can be wind dispersed.

Himalayan knotweed is an herbaceous perennial that rapidly grows by producing new shoots from rhizomes and crowns. New shoots emerge from mid-spring to late summer. Sprouts emerging from the ground are pointed and slender, resembling asparagus shoots.  They may not be hollow until they mature. Flowering occurs in late summer (August to September), with fruit set in September.  

Himalayan knotweed grows fast by way of rhizomes and crowns and reaches full height by late June. Growth is primarily by a dense network of rhizomes that can grow 50 – 65 feet laterally and will produce new shoots. Himalayan knotweed can also reproduce sexually (seed production) by way of perfect flowers (male and female parts in the same flower), although this appears to be rare in North America.  Populations in British Columbia appear sterile.  

Himalayan knotweed is not frost tolerant and it will die back in the winter, but this provides minimal relief for native ecosystems because by dropping its leaves the plant makes a dense mat on the ground that can prevent native species from germinating.  

Himalayan knotweed grows in loamy, silty, or sandy soils. It does require full sun and moist sites. For instance, Himalayan knotweed loves riparian areas, drainage systems, and roadside ditches.  

Himalayan knotweed will start with reddish shoots in the spring (resembling asparagus) and grow to heights of around two metres by summer. In the fall, the leaves turn yellow and wilt, eventually dropping to the ground and leaving hollow stalks by winter.

In winter, the plant appears dead but each spring, new leaves appear, and new shoots emerge around the established radius as the plant spreads via rhizomes. The rate of seasonal and annual growth will be a strong indicator that this is an invasive species.   

Himalayan knotweed has been reported in British Columbia, Nova Scotia, and Newfoundland. There are no known populations in Ontario. 

Himalayan knotweed may occasionally produce seeds that will wind-disperse to new locations, but this appears to be rare in North America. The more common mode of spread occurs by cloning via rhizome and/or stem fragments. Fragments as small as two centimetres can start new infestations by becoming dislodged and moving to new locations where a clone will establish and grow. This is common along waterways and drain systems during flood events but can also occur during any type of infrastructure project that moves vegetation and soil to new locations. Plants can also spread by wind, wildlife, cutting and mowing, and hitching a ride on equipment.  Human actions such as selling, purchasing, and trading plants can also contribute to the spread.  

Ecological Impacts 

Himalayan knotweed will form dense monocultures that shade out native plants and create dense mats of leaf litter. Dense monocultures and leaf litter mats will eliminate native species by reducing germination rates and shading out other plants.

Over time, the infestation leads to a reduction in trees and loss of biodiversity.

Because Himalayan knotweed typically establishes on floodplains and riparian areas, these monoculture stands will also lead to an increase in streambank erosion over time. Knotweed infestations near waterways will decrease the quality of fish and wildlife habitat, by choking out native species and creating impenetrable walls along the water and changing the food web.

Leaves and organic material in the stems are colonized by microscopic algae and bacteria, known as periphyton, that are an important food source for aquatic macroinvertebrates. Because they are not as nutritious, knotweed leaves may have less periphyton, which would support fewer invertebrates – which may in turn negatively impact those invertebrates’ fish predators. However, more research is needed.  

Economic and Social Impacts 

Himalayan knotweed is incredibly strong and can grow directly through concrete and asphalt, damaging infrastructure like buildings, sidewalks and roads, or drainage systems.  This can create significant repair, maintenance, and management costs. Knotweeds have also been known to affect rail beds and compromise train safety. Because Himalayan knotweed can grow so fast, it can create dangerous situations by obstructing sightlines along roadsides, affecting the safety of drivers, bikers, and pedestrians. Recreation and tourism can be impacted when Himalayan knotweed establishes and develops into impenetrable walls along rivers, and creeks impeding access for anglers and boaters and other aquatic recreationists.